Device and method for nodal multiple access into communications channels

ABSTRACT

A nodal division multiple access technique for multiple access to a communications channel such as a satellite transponder. The present invention provides multiple access into a communications channel where each accessing site utilizes one signal from a composite amplitude/phase digital signal constellation, such that demodulators receive the composite signal without changes in the system design related to the multiple access operation.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. application Ser.No. 10/153,250, filed May 22, 2002 now U.S. Pat. No. 7,292,547, which isincorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates generally to a system and method formultiple access to a given transponder and more particularly to multipleaccess in satellite communications.

BACKGROUND OF THE INVENTION

Communications channels, such as the RF communications channelrepresented by a geosynchronous communications satellite transponder,are an important, economically valuable resource. A multitude of schemeshave been developed for efficient modulation and coding of a singlecarrier using a communications channel and for multiple accesstechniques where multiple carriers share a channel. In multiple access,the typical design case involves transmissions from dispersed geographiclocations where any one site has a capacity demand less than the totalcapacity available, but where the aggregate demand from all sites isequal to the total capacity.

In the specific case of satellite communications, multiple access to agiven transponder has been achieved using frequency division multipleaccess (FDMA), time division multiple access (TDMA), code divisionmultiple access (CDMA), and various combinations of these techniques.The FDMA, TDMA, CDMA and variations thereof all require specificequipment features at both the transmit and the receive ends of thesystem.

In a satellite system, including a so-called geostationary system, thesatellite is always moving with respect to the modulators anddemodulators. The geometry and satellite motion causes slowly varyingdistances between the transmit sites and the satellite and between thesatellite and the receive sites. In turn, the varying distance along theline-of-sight causes changes in carrier frequency, and hence phase, dueto Doppler. Since the multiple access signals of interest herein sharethe satellite transponder and the downlink path, the key issues are thedifferences in paths of the uplink signals. These differences areprimarily due to slight differences in the geometry between uplink siteand the satellites and slight differences in signal propagation throughthe atmosphere such as phase scintillation.

These variations present design challenges for conventional multipleaccess techniques such as FDMA, TDMA and CDMA. Any new technique mustalso accommodate the time-varying geometry that occurs in acommunications satellite system.

SUMMARY OF THE INVENTION

The present invention is a system and method for communications signalprocessing including the capability to handle time-varying effects suchas that which occur in a communications satellite system. The presentinvention provides multiple access into a communications channel whereeach accessing site utilizes one signal from a composite amplitude/phasemodulated digital signal constellation, such that the demodulatorsreceive the composite signal without changes in the receiver designrelated to the multiple access operation.

The present invention permits nodal division multiple access (NDMA)where standard modulation techniques are used but where innovativeprocessing at the modulator locations permits multiple carriers to sharea single communications channel, such as a satellite transponder. Withmodem signal processing technology, the implementation at the modulatorend is practical and economical. No changes are necessary in thedemodulators. This is a very important advantage in asymmetricalapplications, such as direct broadcast satellite (DBS), where there aremany more demodulators than modulators.

According to the NDMA technique of the present invention, the userreceivers are generally the same as receivers for certain modulationformats without multiple access. NDMA can directly utilize the majorbody of theory and practice already available in digital communications,particularly in amplitude phase shift keying (APSK). NDMA can be used asa network evolution technique in that all deployed receivers would havethe capability to demodulate any appropriate signal, but as the networkevolves the transmitted signal becomes a multiple access composite ofsignals from different geographic points.

As a specific application, the invention could be utilized in anadvanced system for DBS re-broadcast of local television signals. Inexisting systems, the local television signal is transported, byterrestrial means, to a small number of major satellite uplink sites.Each of the uplink sites aggregates channels into groups matching thecapacity of a single satellite transponder. Each uplink carrier then isa “single access” carrier from one of the major satellite sites to adirect broadcast satellite (DBS) transponder.

According to the NDMA system and method described herein, the localchannels can be uplinked from less expensive sites nearer to thetelevision signal's point of origin. For example, the capacity of atransponder could be shared between two local television markets with anuplink in each market. The NDMA invention eliminates the terrestrialtransmission costs to a more complex, distant uplink facility.

It is an object of the present invention to utilize PDMA in satellitecommunications. It is another object of the present invention to utilizePDMA as a network evolution technique. It is still another object of thepresent invention to permit a new signal to be uplinked from a newgeographic site distant from an original uplink site.

A further object of the present invention is to provide a method formaking a transmitted signal become a composite of multiple signals fromdifferent geographic sites.

Other objects and advantages of the present invention will becomeapparent upon reading the following detailed description and appendedclaims, and upon reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this invention, reference shouldnow be had to the embodiments illustrated in greater detail in theaccompanying drawings and described below by way of examples of theinvention. In the drawings:

FIG. 1 is a diagram of a static geometry system having a terrestrialrepeater;

FIG. 2 is a diagram of a static geometry local modulating subsystem;

FIG. 3 is a system diagram of a dynamic geometry system;

FIG. 4 is a diagram of a dynamic geometry local modulating subsystem;

FIG. 5 is a signal constellation of a composite input signal accordingto the present invention;

FIG. 6 is a signal constellation of the composite input signal afterremoval of the local signal according to the present invention;

FIG. 7 is a signal constellation of an estimate of distant signalcoordinates according to the present invention;

FIG. 8 is a signal constellation of local coordinates according to thepresent invention;

FIG. 9 is a signal constellation of the output signal according to thepresent invention; and

FIG. 10 is a system diagram of an alternative embodiment of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Some background material and terms are defined herein for the detaileddescription of the preferred embodiments of the present invention. A“local transmit site” is one of several sites which transmit signals inmultiple access with other distant sites into a common channel. Most ofthe required signal processing is described herein with regard to asingle “local” site. Each local site transmits a modulated “carrier” or“signal” which, after the common channel, is a part of the “composite”signal. The term “carrier” denotes an unmodulated transmission.

In-phase (I) and quadrature (Q) carrier and signal components areutilized throughout the present invention. Each local signal isdescribed by its location in the I-Q coordinate space of the underlyingcarrier.

The following description applies to a 4PSK satellite communicationsapplication. With a 4PSK (QPSK) modulation design, the present inventionpermits multiple access from two locations, with each transmitting a2PSK signal. Other applications can be generalized from this particularcase. For example, with 16QAM, the invention permits NDMA from twosites, each with 4PSK modulation or from as many as four sites each with2PSK or 4PSK modulation. It should also be noted that the fundamentalsof the invention apply to any system and modulation where themodulations can be combined within the channel and where the distantcarriers can be appropriately controlled. For example, the presentinvention applies to amplitude and phase modulations if the linearity ofthe communications channel permits quasi-linear superposition of themodulations from the different transmit locations.

A variety of forward error control (FEC) techniques may be used with thesignals described herein. For the NDMA system described herein, the FECdesign is largely independent and, in fact, an important feature of theinvention in that the FEC scheme may be different for each accessingsignal. For example, an NDMA system may start with a 4PSK signal withone FEC scheme and then later add a 2PSK signal with a different, moreadvanced FEC.

FIG. 1 is a block diagram of a static geometry terrestrial repeatersystem 10 using 4PSK for multiple access to a terrestrial repeater. Thepresent invention is applicable to static geometry applications, such asa terrestrial repeater. Another example of a static geometry applicationis a satellite whose motion is not significant and the Doppler effect isnegligible. There are two originating sites, site A and site B. Each ofthe two originating sites transmits 2PSK to a transponder 12.Transponder 12 transmits a composite signal, A+B, to Site A, Site B, anda plurality of other, much simpler, receiving sites 14. The compositesignal A+B is also used as a frequency and phase reference signalaccording to the present invention.

FIG. 2 is the local modulating subsystem 20 for a static example, whichis located at either Site A or Site B. The incoming composite signal A+Bis received at a M-ary receiver 22 where it is used as a timing,frequency and phase reference. The composite signal A+B is recoveredfrom the transponder and converted into In-phase and Quadraturecomponents. Ambiguity resolution is performed to determine a referencefor the phase of the signal. The converted recovered carrier and thephase ambiguity indicator are sent to the modulator, where theinformation bit stream is also fed. The outgoing modulated signal isoutput from the modulator.

The static geometry application shown in FIGS. 1 and 2 is notparticularly challenging for the present invention of NDMA, because thegeometry is fixed and the differential propagation impairments for thepaths A and B are small.

FIG. 3 is a block diagram of a system 30 with a time-varying, dynamicgeometry involving an orbiting communications satellite 32. The relativemotion of the satellite 32, and transponder 34, is shown by a dashedline 33 with arrows indicating the direction of motion. Transmit site A36 transmits signal A to the transponder 34. Transmit site B 38transmits signal B to the transponder. A composite signal A+B istransmitted from the transponder back to each of sites A 36 and B 38.The composite signal A+B is also transmitted to a plurality of receivesites 40. The signals are transmitted and received through a variety ofatmospheric effects 42.

The application shown in FIG. 3 introduces two new effects to providesources of timing, frequency and phase instability. In contrast to thestatic geometry application shown in FIG. 1, the satellite applicationin FIG. 3 introduces variations in the signal. Because the satellite 32is not perfectly “geostationary”, a small relative motion with respectto the transmit sites 36, 38 gives timing, frequency and phase changes.Also, since the different uplink signals do not follow the same pathsthrough the atmosphere, it is assumed that small differential delays, orphase changes, will occur.

FIG. 4 is a block diagram of the local modulating subsystem 50 for thedynamic geometry system of FIG. 3. FIG. 4 shows the signal processingused at each of the transmit sites. With reference to FIG. 4 it shouldbe noted that 4PSK modulation is shown, however, with refinements thearchitecture shown may be generalized and applied to amplitude/phaseshift keying such as 16QAM. In the description with respect to FIGS. 4through 9, the concept of a “local I/Q coordinate space” is used tocollect incoming signal measurements and synthesize outgoing signals. Itshould be noted that the I/Q space is only a localized signal processingimplementation and it does not directly relate to the composite signalconstellation. The local I/Q coordinates are the in-phase andout-of-phase components of a given signal with respect to the local VCOreference.

Referring to FIG. 5, a signal constellation 100 for the composite signalA+B is shown. The composite signal A+B is input to the local modulatingsubsystem 50 of FIG. 4. The local modulation is removed 52 from thecomposite signal A+B. This is accomplished by coarse and finesynchronization processes wherein a replica of the outgoing signal 54 issubtracted 56 from the composite signal A+B. The synchronizationprocesses drive toward minimization of the difference of the signals andhence provide a “clean” replica of the distant signals. The coarse partof the synchronization process is open loop and uses an archive 58 ofthe outgoing signal 54 and parameters of the satellite orbit andsatellite to local uplink geometry.

Using the orbit parameters and geometric data, straightforwardcalculations provide an estimate 60 of the satellite-to-ground distance,and hence signal delay and the time-rate-of-change of the distance andhence the timing and frequency shift. Since the roundtrip delay is lessthan 300 msec, the signal archive storage requirements are modest. Anexample of a fine tracking loop is an early/late delay-locked loop thatremoves 52 local modulation from the composite signal A+B. The loop isof the type used in spread-spectrum communications systems, an exampleof which is described in Digital Communications and Spread SpectrumSystems, R. E. Ziemer and R. L. Peterson, Macmillan, 1985, specificallyat Chapter 9, pages 419 through 483, which is incorporated herein byreference. FIG. 6 is a signal constellation 102 of the signal after thelocal signal has been removed.

Referring back to FIG. 4, the coarse blocks 58, 60 and the fine block 52will maintain lock for reliable operation except, perhaps, immediatelyfollowing a satellite orbital maneuver. Therefore, in terms of customersatisfaction, it is suggested that orbital maneuvers be carried outduring low customer interest, such as the early morning hours, tominimize the impact on perceived system availability.

The carrier A+B is recovered 62 from the input modulation. The carrieris a composite of the carriers from the plurality of distant sites.Since all of the uplink sites have this circuitry, all will continuouslydrive towards a common frequency. Bit decisions are not made at thissignal processing stage. Signal samples are output in the local I/Qcoordinate space. FIG. 7 is a signal constellation 104 of the distantsignal coordinates that are estimated.

Referring back to FIG. 4, signal samples are examined and at each bittime, the optimum location for local signals is recomputed 64 in localI/Q coordinates for the local M-ary signals. An algorithm is used tomaximize the distance between the local signals and the signals receivedfrom distant sites. Since all uplink sites have the same circuitry, theyall will continuously drive toward an optimum signal constellation. Forexample, FIG. 8 shows boundaries 106 for the signal constellation. Ifthe received distant signal shown in constellation 104 of FIG. 7 isslowing rotating in the local I/Q space, the algorithm will cause theoutgoing modulation to rotate appropriately to maintain roughly thedistance intended in the M-ary modulation design shown by the boundaries106 in FIG. 8.

Referring again to FIG. 4, orbital parameters and geometric data areused to pre-distort or compensate the outgoing signal 66 such that theeffects of the changing geometry are removed 68. The local signalconstellation 108 is shown in FIG. 9. Referring back to FIG. 4, thecompensated signal 68 and the I/Q coordinates of the optimum location 64and the local bit stream for transmission 72 are used to create 70 theoutgoing modulated signal. The I/Q references from the local voltagecontrol oscillator as modified at block 68 are used to create 70 theoutgoing phase modulated signal. The implementation described withreference to FIG. 4 occurs at each uplink site. For example, with 16QAM,the implementation of FIG. 4 would be deployed at up to four sites, witheach site transmitting a 2PSK signal.

In an alternative embodiment, shown in FIG. 10, there is no need for anoutgoing signal archive. In the embodiment shown in FIG. 10 aninitiating sequence 80 is implemented. Site A is the “master” site 82and transmits first. Site B, called the “slave” site 84, receives thesignal from site A by way of the satellite. Site B demodulates 86 thesignal from site A 82 to obtain symbol timing and signal carrierfrequency information about the signal from site A. Slaving to thisinformation, site B then transmits 88 its own signal with synchronizedsymbol timing and a carrier frequency equal to the received frequency.The two signals make a new composite signal. For example, site Atransmits a 4 PSK signal and site B transmits a 4 PSK signal, theircombination produces a 16 QAM signal as a composite signal.

Upon receiving the composite signal, Site B continuously tracks 90symbol timing and carrier frequency errors between the two signals witha phase locked loop 92 to line up symbol timing and carrier phase withrespect to the signal transmitted by Site A, by optimizing the placementof individual signal nodes within the composite constellation.Synchronization of symbol timing and carrier frequency/phase ismaintained with the phase locked loop. In this regard, there is no needto store signals for cancellation. It is possible that additional slavesites be sequentially added to transmit higher-order modulation signals.

The invention covers all alternatives, modifications, and equivalents,as may be included within the spirit and scope of the appended claims.

1. A system for nodal division multiple access communication signalprocessing comprising: a single satellite transponder; a plurality ofmodulated signals transmitted to said satellite transponder from atleast first and second sites; a composite signal of said plurality ofsignals transmitted to at least one receive location; a modulatingsubsystem for modulating a carrier of said composite signal with aninformation bit stream by removing local modulation from said incomingcomposite signal, determining an estimate of signal delay time andcarrier frequency shift of the composite signal, using said estimate tomodify an outgoing signal thereby defining a replica of said outgoingsignal, and subtracting said replicated signal from said compositesignal, said outgoing signal being selected from stored outgoingsignals; a transmitter for transmitting said outgoing signal from saidat least first and second sites.
 2. The system as claimed in claim 1further comprising a local carrier reference defined by said compositesignal for frequency and phase reference at one of said at least firstand second sites.
 3. The system as claimed in claim 2 further comprisinga local I/Q coordinate space defined by in-phase and quadraturecomponents of said composite signal, wherein said local I/Q coordinatespace is used for collecting measurements of said composite signal andsynthesizing outgoing signals.
 4. The system as claimed in claim 3further comprising a fine tracking loop for tracking differences in saidcomposite signal.
 5. The system as claimed in claim 4 wherein said finetracking loop further comprises an early/late delay locked loop.
 6. Thesystem as claimed in claim 3 further comprising an optimum I/Q locationfor said plurality of modulated signals.
 7. The system as claimed inclaim 6 further comprising an algorithm for maximizing a distancebetween signals in said composite signal, a local signal and a signalfrom at least a second site.
 8. A system for nodal division multipleaccess to a satellite communications channel comprising: a modulationscheme having an M-ARY phase/amplitude signal constellation; a firstmodulated signal transmitted from a first site to a transponder; asecond modulated signal transmitted from a second site to saidtransponder; a composite signal of said first and second modulatedsignals transmitted to at least one receive location; local modulationbeing removed from said transmitted composite signal at at least one ofsaid at lest two transmit sites; a local I/Q coordinate space defined byIn-phase and Quadrature components of said composite signal; an optimumI/Q location for said composite signal determined by maximizing adistance between signals in said composite signal, a local signal and asignal from said at least a second site; geometric data used to removeeffects of a changing geometry for said transponder from said compositesignal; a modulator for modulating said composite signal with aninformation bit stream to define an outgoing modulated signal; and atransmitter for transmitting said outgoing modulated signal.
 9. Thesystem as claimed in claim 8 further comprising a replica signal of saidoutgoing modulated signal determined by an estimate oftransponder-to-ground distance, signal delay time, time-rate-of-changeof said transponder-to-ground distance, and frequency shift wherein saidreplicated signal is subtracted from said composite signal.
 10. A methodfor multiple access to a communications channel on a satellitecomprising the steps of: transmitting a first modulated signal from afirst site to a transponder; transmitting at least a second modulatedsignal from at least a second site to said transponder; transmitting acomposite signal of said first and second signals to at least onereceive location; modulating a carrier of said composite signal with aninformation bit stream by removing local modulation from said incomingcomposite signal, determining an estimate of signal delay time andcarrier frequency shift of the composite signal, using said estimate tomodify an outgoing signal thereby defining a replica of said outgoingsignal, and subtracting said replicated signal from said compositesignal, said outgoing signal being selected from stored outgoingsignals; and transmitting said outgoing signal from said first site andsecond site.
 11. The method as claimed in claim 10 further comprisingthe step of using said composite signal as a local carrier reference forfrequency and phase reference at one of said at least two transmitsites.
 12. The method as claimed in claim 11 further comprising thesteps of: collecting composite signal measurements; and synthesizingoutgoing signals.
 13. The method as claimed in claim 12 furthercomprising the steps of: recovering a frequency of said compositesignal; and converting said recovered composite signal into In-phase andQuadrature components thereby defining a local I/Q coordinate space forsignal processing at one of said at least two transmit sites and whereinsaid local I/Q coordinate space is determined with respect to a localvoltage control oscillator reference.
 14. The method as claimed in claim10 further comprising the step of applying a fine tracking loop fortracking differences in said composite signal.
 15. The method as claimedin claim 14 wherein said fine tracking loop further comprises anearly/late delay locked loop.
 16. The method as claimed in claim 13wherein said step of modulating said local carrier reference furthercomprises determining an optimum I/Q location for said modulatingsignal.
 17. The method as claimed in claim 16 further comprising thestep of maximizing the distance between signals in said compositesignal, a local signal and a signal from said at least a second site.18. The method as claimed in claim 10 wherein said step of transmittingan outgoing signal further comprises the step of compensating saidoutgoing signal to remove the effects of a changing geometry for saidsatellite.
 19. The method as claimed in claim 10 further comprising thesteps of: receiving said first modulated signal at said at least asecond site; obtaining a frequency of said first modulated signal; andwherein said step of transmitting a second modulated signal furthercomprises generating a carrier signal having a frequency equal to saidfrequency of said first modulated signal.
 20. A nodal division multipleaccess method for multiple access to a satellite communications channelcomprising the steps of: utilizing a modulation scheme having an M-ARYphase/amplitude signal constellation; transmitting a first modulatedsignal from a first site to a transponder; transmitting at least asecond modulated signal from at least a second site to said transponder;transmitting a composite signal of said first and second modulatedsignals to at least one receive location; removing, at one of said atleast two transmit sites, local modulation from said incoming compositesignal; recovering a frequency of said composite signal; converting saidcomposite signal into In-phase and Quadrature components, therebydefining a I/Q coordinate space for signal processing at one of said atleast two transmit sites and; determining an optimum I/Q location forsaid composite signal by maximizing a distance between signals the firstmodulated signal and the second modulated signal in the I/Q coordinatespace; using geometric data to remove effects of a changing geometry forsaid transponder from said composite signal; modulating said compositesignal with an information bit stream to define an outgoing modulatesignal; and transmitting said outgoing modulated signal.
 21. The methodas claimed in claim 20 wherein said step of removing local modulationfurther comprises the steps of: creating a replica of said outgoingsignal; and subtracting said replicated signal from said compositesignal.
 22. The method as claimed in claim 21 wherein said step ofreplicating said outgoing signal further comprises the steps of:determining an estimate of transponder-to-ground distance, signal delaytime, time-rate-of-change of said transponder-to-ground distance, andfrequency shift; and using said estimate to define said replicatedsignal.